Broadly, the invention relates to a machine for continuously mixing solid particles with a fluid composition. More specifically, the machine is used as a blender in which sand or sand-like particles are mixed or blended with a gel composition, and the resulting slurry is pressurized by the blender itself. Typically, the slurry is used to treat a well in a petroleum recovery operation.
The blender machine described in U.S. Pat. No. 4,453,829 (Althouse) is typical of conventional blender machines now being used in oil or gas recovery operations. This machine has a slinger element of a toroidal configuration with a concave upper surface. Several upstanding blade members are mounted on the concave surface of this slinger and an impeller member is attached to the underside of the slinger. The slinger and the impeller are enclosed within a housing and fastened to the end of a drive shaft rotated by a motor mounted above the housing. A hopper is mounted above an inlet eye in the top of the housing, for introducing sand or other solid particles into the housing. At the bottom of the housing is a suction eye inlet, for drawing fluid into the housing, and the resulting fluid-solid mixture is discharged through an outlet port in the housing.
In the operation of the blending machine described above, sand flows out of the hopper in a continuous stream and drops onto the rotating slinger through the inlet eye in the housing. With the impeller and slinger rotating at the same speed, the vortex action of the impeller creates a suction force that draws the gel composition into the casing through the suction eye inlet. As the gel is pulled into the casing it is pressurized by the impeller and it mixes throughly with the sand being flung outwardly, in a centrifugal action, from the slinger. The sand-gel mixture is then continuously discharged, under pressure, through the outlet port, from which it is carried into a pumper unit and injected into a well.
The Althouse blender has a major drawback that makes it difficult for this machine to thoroughly mix a slurry of a particulate material, such as sand, and a fluid, such as a gel composition. The problem is caused by air in the sand, which becomes entrained in the fluid during the mixing operation. In a typical mixing operation, for example, the slinger and impeller may be rotated at speeds of up to 1,000 rpm. At these high speeds, the centrifugal action of the slinger causes the sand particles to move outwardly from the slinger into the whirling slurry mass that lies between the slinger-impeller units and the housing and below the impeller.
Centrifugal forces in the whirling slurry set up a radial pressure gradient, and since the density of air is much less than that of the slurry composition, the air is forced toward the center of the slurry mass. Therefore, any air that the sand carries below the upper edge of the slinger can't move outwardly against the pressure gradient and return to the area above the slinger. In other words, once the air moves downwardly from the slinger, it can't reverse its direction and "break out" of the slurry composition through the inlet eye at the top of the housing. Similarly, since the impeller increases in diameter as you move away from the slinger-impeller interface, air can't travel downwardly from the interface toward the underside of the impeller. Since the Althouse blender has no way to exhaust the air, it accumulates at the slinger-impeller interface. Because the slinger is larger in diameter than the impeller, the accumulated air "overflows" from the interface region into the region below the impeller. As a result, some of this air collects below the center of the impeller and forms an "eye". Since the impeller can't pump air, the Althouse blender rapidly loses its suction pressure and it ceases to pump the slurry mixture through the discharge outlet in the housing.
The present invention is based on a modification of the Althouse blender that solves the air entrainment problem. In the machine of this invention the entrained air in the slurry mixture can break out of the fluid phase through some interior and exterior air exhaust spaces and channels that are built into the modified blender structure.